According to Birds in Backyards, birds can fly because every part of their anatomy has evolved to enhance flight. In order for a bird to fly, it must be lightweight. Therefore, birds have evolved lightweight hollow bones which resemble a honeycomb, making them strong yet light. In addition, birds have fewer organs and no teeth, and they use a gizzard to grind up their food.
One adaptation birds have for flight are their wings. To get power for flight, birds must flap their wings by contracting their large breast muscles, which are anchored to a keel on their sternum. Many birds use the air currents to soar and glide, while other species must continuously flap their wings to hover. However, not all birds fly. Some birds have evolved to be flightless in the absence of predators or to enable them to swim, which is the case for penguins, states Birds in Backyards. Also, a bird's body temperature is around 104 degrees Fahrenheit, which is higher than the temperature of an average mammal, according to Birds in Backyard. This is due to their insulating feathers and a layer of fat that can be found in certain species. Their higher body temperature allows the cells in their muscles to work 2.2 times faster, and as a result, allows their muscles to relax more rapidly, explains Birds in Backyards.